The present invention relates to a method for increasing the braking effect in a motor vehicle with a hydraulic brake system, wherein in case of a reduced effectiveness of the brake system, a brake pressure in at least one wheel brake is increased by switching on an energy supply unit.
In addition, the invention relates to a device that is suitable to implement the method.
When subjected to high thermal load, under the influence of saline solution and major wear, hydraulic brake systems exhibit a reduced effectiveness so that, even given a high initial pressure in the master brake cylinder that is adjusted by the driver, the brake system will not bring about sufficient deceleration of the vehicle. The reduced braking effect at high working temperatures of the brake system is referred to as ‘fading’.
In the cases illustrated hereinabove, the reduced braking effect of the vehicle brake system results, however, especially from a reduced coefficient of friction between the brake pads and the brake disc, so that a special rate of brake pressure is required in order to brake the vehicle effectively.
German published application DE 102 38 427 A1 discloses a method of equal type, wherein in case of limited effectiveness of the vehicle brake, enabling of an energy supply unit causes increase of the brake pressure or the brake force in the wheel brakes because hydraulic fluid is pumped out of the master brake cylinder into the wheel brakes. A method of this type is also referred to as FBS function (FBS: Fading Brake Support).
The energy supply unit is activated in the FBS function when the initial pressure in the master brake cylinder adjusted by the driver exceeds an activation threshold, which is determined depending on the vehicle deceleration. Thus, the energy supply unit is enabled when the actual vehicle deceleration is by a defined value lower than the vehicle deceleration that is expected due to the adjusted initial pressure, with the brake system functioning properly.
The energy supply unit is usually disabled when the initial pressure in the master brake cylinder drops below a predetermined threshold value, since it is assumed in this case in particular that the driver has released the brake pedal and, thus, terminated the brake operation.
However, the problem encountered in this respect is that frequently so high a quantity of hydraulic fluid is pumped from the master brake cylinder into the wheel brakes that a drop in pressure occurs in the master brake cylinder, without the driver releasing the brake pedal.
This will disable the energy supply unit, although the driver continues with the brake operation. With a continuous brake application by the driver, the energy supply unit is switched on again when the pressure in the master brake cylinder has increased beyond the activation threshold after the supply of brake fluid from the wheel brakes. In total, however, the result is a reduced average braking effect and thus a longer stopping distance.
Besides, the driver will notice the pedal reaction and the noise developing during very frequent enabling and disabling operations of the energy supply unit by a higher frequency and, thus, in an unpleasant way.
In view of the above, an object of the invention is to overcome the illustrated shortcomings of the state of the art, and, in doing so, to further develop in particular a method of the type mentioned in the beginning and a device for implementing the method in such a way that the stopping distance is shortened.